adding tops very slow

[MTLehner]

adding multiple tops together is very slow. Especially for easy task like multiplying a top for a condensed phase simulation this is not necessary. A lot of overhead could be avoided by adding a multiply option.

• top1.multiply(512) (multiply one top 512 time = add 511 copies of itself)
• top1 *= 512 (should do same thing as above)

This would significantly speed up the creation of topologies